The invention relates to a tachometer signal
invention relates conditioning circuit for an internal combustion engine having a battery circuit and an ignition coil with a primary low voltage lead and a secondary high voltage lead.
The invention arose during efforts to develop a reliable and widely usable tachometer for marine drive engines, including racing applications. In high performance marine engines, the use of nonsuppression spark plugs and leads have caused tachometer problems, particularly RFI (radio frequency interference), EMI (electromagnetic interference), and other transients. The present invention addresses and solves this problem.
The invention adapts most standard tachometers to most ignition coils. The invention converts the ignition pulse on the primary lead of the coil to a tachometer signal transitioning between ground and a voltage level in the range of battery voltage, which signal is readily usable in most standard tachometers. The ignition coil converts a low voltage such as 140 volts on its primary lead to a high voltage such as thousands of volts on its secondary lead supplying spark plug voltage. The low voltage on the primary lead, e.g. 140 volts, is still too high for use in standard low voltage tachometers. The present invention allows the primary lead voltage to operate a low voltage tachometer, e.g. less than 12 volts.
The invention cleans up noisy signals such as caused by point bounce in breaker point systems. The invention eliminates the above noted RFI and EMI problems. When used with a high performance marine battery capacitor discharge ignition system, the invention eliminates the requirement for a special reverse polarity tachometer and allows the use of a standard tachometer connected to the 140 volt primary lead of the ignition coil that has an extremely fast rise time and short duration. The invention enables a single standard tachometer to be left in the dash if engines and ignition systems are changed. The invention also allows for usage of various makes of tachometers because of the readily usable tachometer input signal provided by the invention.